function realu = U3(x,t)
        s = 4*pi^4/5*ones(size(x));
        ssign = 1;
        for k = 1:201
          s = s + ssign*8*(pi^2/k^2-6/k^4)*exp(-k^2*t)*cos(k*x);
          ssign  = -ssign;
        end
        realu = s;
end
